In recent years, attention has been focused on quantum information processing such as a quantum cryptography system, quantum teleportation, and a quantum computer, and quantum lithography using the principles of quantum mechanics (for example, C. Bennet and P. Shor, IEEE Trans. Inf. Theory 44, p. 2724 to 2742 (1998), and A. Boto, P. Kok, D. Abrams, S. Braunstein, C. Williams, and J. Dowling, Phys. Rev. Lett. 85 (13), p. 2733-6 (2000)). For these, a quantum correlated photon pair is used which is so called “quantum entanglement.” This phenomenon called quantum entanglement is that a single photon (pumped light) of high energy is split into two photons of low energy (a signal photon and an idler photon) to cause these two photons to interfere with each other for correlation.
For a method of generating the quantum correlated photon pair, for example, a method is known in which a parametric crystal (for example, BBO (β-Ba2B2O4) crystal) that generates and amplifies parametric fluorescence is utilized to launch a pumped light, a pair of two photons of a signal photon and an idler photon is generated at the same time by a spontaneous parametric down-conversion (SPDC) process, and a non-polarizing beam splitter is used to produce a quantum entangled state (H. Fearn and R. Loudon, J. Opt. Soc. Am. B 6, p. 917-927 (1989)).
In other words, for example, when a pumped light (ωp is a frequency, and kp is a wave number (k is a vector)) is launched into a Type II parametric crystal that generates and amplifies parametric fluorescence, a pair of two photons (a signal photon (ωs, ks) and an idler photon (ωi, ki)) is generated at the same time which has energy lower than that of the launched pumped light by parametric down-conversion in the parametric crystal. The generated pair of two photons is emitted as a parametric fluorescence pair in an orthogonal polarization state from the parametric crystal at the same time along two non-concentric cones.
As shown in FIG. 1(a), a phase matched parametric fluorescence pair exists only at a position of point symmetry to beams of pumped light (in the drawing, a black dot located between rings) (ks and ki). Then, only those satisfying the following phase conditions are cut out by a pin hole among the fluorescence pairs, lead to a non-polarizing beam splitter, and then caused to interfere with each other to form a quantum correlated photon pair with entangled paths.ωp=ωs+ωi kp=ks+ki (Where subscripts p, s, and i indicate a pumped photon, a signal photon, and an idler photon, respectively.)